1. Field of the Invention
The invention relates to the field of boot assembling, and relates more particularly to boots that comprise a rigid sole and a flexible upper.
2. Description of Background and Relevant Information
These boots are generally used in gliding, walking, or rolling sports, such as snowboarding, skiing, hill-walking, roller skating, biking or the like.
Indeed, these sports require that the user be able to take reliable supports with the foot, while maintaining a minimum comfort and mobility of the leg.
A rigid sole makes it possible, for example, to retain a boot on a gliding apparatus such as a snowboard or a ski, or to reduce the fatigue of the user's foot by avoiding contraction of the muscles of the arch of the foot, especially in the case of point/punctual support of the sole on the ground.
A flexible upper enables the user to move the legs to manage his balance or operate the gliding apparatus.
However, regardless of the mode or field of utilization of this type of boot, the manufacture requires an affixation of the sole and of the upper. The affixation is done conventionally by gluing or by injection, because there is currently no machine that is capable of sewing a flexible upper on a rigid sole.
A classic example is that of boots having a flexible upper. The upper is mounted on a rigid and glued form. The sole is then positioned on the upper, and a pressure is applied between the sole and the form so as to perform the adhesion. The flexibility of the sole enables the latter to properly press on the upper mounted on the form. The quality of the adhesion essentially depends on the pressure applied during this operation.
The problem is more complex in the case of boots with a rigid sole. It is impossible to use the traditional gluing technique because the sole cannot deform sufficiently to press on the upper, regardless of the pressure used.
The solution then consists of using an injection technique explained hereinafter.
An upper is mounted on a form to constitute an assembly positioned within a mold that has the dimensions of the boot to manufacture. A volume that corresponds substantially to the sole of the boot remains between the walls of the mold and the assembly. This volume is filled by a plastic material hot injected at high pressure, generally comprised between 100 and 200 bars. After cooling, the plastic material has the aspect of a rigid sole affixed to the flexible upper: the boot is completed.
However, this injection technique has numerous disadvantages.
Certain disadvantages come from the fact that the volume to be filled by the plastic material to make the sole is not constant; indeed, the manufacturing tolerances for a flexible upper are substantial because of the stresses related to the construction, and a flexible upper is capable of being compressed under the effect of the injection pressure. As a result, the affixation of the rigid sole to the flexible upper is imperfect and/or irregular. It also follows that the volume available for the user's foot is reduced randomly, which has the effect of creating discomforts and traumatisms during use.
It also follows that the physical properties of the sole are often considerably altered when the pressure drops too much during injection. Indeed, the pressure is maintained at a sufficient level only in the case where the volume to be filled is constant, by means of the press technology.
Other disadvantages of the injection technique are related to scaling factors. First, a mold is needed for each boot size, whose manufacture is often time-consuming and costly, and whose amortization imposes mass production.
Furthermore, any modification of a boot leads to an equally costly modification of the mold.
The pressurized injection of the plastic requires adequate equipment.
The implementation of such a process is complex and requires a sophisticated equipment, as well as qualified personnel.